Method for preparing a dialkyl carbonate, and its use in the preparation of diaryl carbonates and polycarbonates

ABSTRACT

Unexpected corrosion of downstream sections of a dialkyl carbonate manufacturing apparatus has been traced to alkyl chloroformate impurities, which slowly decompose to yield hydrochloric acid. An improved process and apparatus for dialkyl carbonate synthesis reduce corrosion by physically removing or chemically decomposing the alkyl chloroformate impurities within the corrosion-resistant upstream sections of the apparatus.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part of U.S. patentapplication Ser. No. 09/682,285 filed Aug. 14, 2001, and U.S. patentapplication Ser. No. 09/682,286 filed Aug. 14 2001.

BACKGROUND OF INVENTION

Polycarbonate resins are useful materials valued for their physical andoptical properties. Methods for the preparation of polycarbonate resinsinclude interfacial processes and melt processes. In interfacialprocesses, as described, for example, in U.S. Pat. No. 4,360,659 toSikdar, a bisphenol is reacted with phosgene in the presence ofsolvents. In melt processes, as described, for example, in U.S. Pat. No.3,153,008 to Fox, a bisphenol is reacted with a diaryl carbonate. Meltprocesses are presently preferred because they avoid the use of phosgeneand solvents.

Use of a melt process for polycarbonate synthesis requires anindustrially efficient process for producing diaryl carbonates. Thereare several known processes for producing diaryl carbonates. One exampleof such a process is described by U.S. Pat. No. 4,182,726 to Illuminatiet al. In this process, diaryl carbonates are produced by reactingdialkyl carbonates with aryl hydroxides (see Scheme I, below).

U.S. Pat. No. 4,182,726 also demonstrates that diaryl carbonates can bereacted together with dihydric phenols to produce polycarbonates (seeScheme II, below).

A preferred process for making dialkyl carbonates is illustrated inScheme III, below, and described, for example, in U.S. Pat. No.5,527,943 to Rivetti et al.; and U.S. Pat. Nos. 4,218,391 and 4,31 8,862to Romano et al.

U.S. Pat. No. 5,527,943 (the '943 Patent) also describes a knowndrawback of the dialkyl carbonate process according to Scheme (III): itproduces water as a by-product. Also, hydrochloric acid (HCl) may becontinuously added to the reaction mixture to maintain a desired molarratio of chloride to copper. Therefore, HCl, CuCl catalyst, and waterare typically found in the stream exiting the reactor vessel.Hydrochloric acid and copper chlorides are very corrosive in thepresence of water, so equipment made from corrosion-resistant materials,such as glass-lined vessels, must be used in the reaction section of achemical plant making dialkyl carbonates by this process. Ascorrosion-resistant equipment is expensive, there is a desire to use itin as little of the plant as possible.

A typical plant for performing preparing dialkyl carbonates according toScheme III may contain three sections: a reaction section for convertingraw materials to dialkyl carbonate, a separation section for isolatingthe dialkyl carbonate from unreacted monomers and by-products, and apurification section for removing water and further isolating thedialkyl carbonate. The '943 Patent teaches that one can minimize theamount of corrosion-resistant equipment required by removing the HClfrom the process stream immediately after the reaction section. Thiseliminates the necessity of using expensive corrosion-resistantmaterials in the separation and purification sections of the plant. The'943 Patent further suggests that removal of HCl and possible copperhalide salts from the stream immediately after the reaction section canbe accomplished by exposing the gas-liquid mixture produced by thereaction to a liquid stream consisting of one of the process fluids. The'943 Patent also states that the operating conditions employed arepreferably adjusted such that the gaseous mixture from the reactor doesnot condense, or condenses only to a negligible extent, before the acidremoval section in order to avoid the necessity of having to reheat themixture before removing the HCl (col. 3, lines 17-30).

In view of the above, it was desirable to construct a plant wherein theHCl and any copper halide salts would be removed from the stream afterthe reaction section to avoid corrosion in the separation andpurification sections. However, a technique similar to that described bythe '943 Patent—removing HCl and copper salts by treatment of avaporized feed in a column using a counterflowing azeotrope fluid fromthe reaction mixture—failed to prevent corrosion in the downstreamseparation and purification sections.

There is therefore a need for a dialkyl carbonate process thatrecognizes and eliminates additional sources of corrosion.

SUMMARY OF INVENTION

The above-described and other drawbacks and disadvantages of the priorart are alleviated by a method of preparing a dialkyl carbonate,comprising: reacting an alkanol, oxygen, carbon monoxide, and a catalystto form a mixture comprising a dialkyl carbonate, an alkylchloroformate, hydrochloric acid, water, carbon dioxide, and carbonmonoxide; and removing alkyl chloroformate from said mixture.

After considerable effort, the present inventors have discovered thatdialkyl carbonate synthesis can form alkyl chloroformate by-productsthat lead to problematic corrosion. For example, in the reaction ofmethanol, carbon monoxide, and oxygen to form dimethyl carbonate(hereinafter “DMC”), methyl chloroformate (hereinafter “MCF”) may beformed as a by-product. The MCF may pass through the HCl removal columninto the separator and purification sections, where it reacts slowlywith methanol and/or water to form corrosive HCl. Therefore, it wasdetermined that steps were needed to remove MCF prior to the separationand purification sections.

Other embodiments, including an apparatus for preparing dialkylcarbonates, are described below.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagrammatic view of a first embodiment of the apparatus.

FIG. 2 is a simplified diagrammatic view of a comparative apparatus thatis susceptible to corrosion.

FIG. 3 is a simplified diagrammatic view of an embodiment of theapparatus in which the fluid passageway 110 comprises two holdingvessels 120.

FIG. 4 is a simplified diagrammatic view of an embodiment of theapparatus in which the fluid passageway 110 comprises four holdingvessels 120.

FIG. 5 is a simplified diagrammatic view of an embodiment of theapparatus in which the fluid passageway 110 comprises a tubular section130.

FIG. 6 is a simplified diagrammatic view of an embodiment of theapparatus comprising an ion exchange resin bed 190.

FIG. 7 is a simplified diagrammatic view of an embodiment of theapparatus in which the fluid passageway 110 comprises a first gas-liquidseparator 90 and a second gas-liquid separator 100.

FIG. 8 is a simplified diagrammatic view of an embodiment of theapparatus in which the fluid passageway 110 precedes the firstgas-liquid separator 90.

FIG. 9 is a simplified diagrammatic view of an embodiment of theapparatus in which the fluid passageway 110 follows the azeotrope column180.

FIG. 10 is a plot of chloride concentrations at the bottom of anazeotrope column 180 as a function of apparatus type (FIG. 2 and FIG. 3)and time.

FIG. 11 is a plot of methyl chloroformate concentrations entering andexiting the fluid passageway 110 as a function of time for an apparatuscorresponding to FIG. 3.

DETAILED DESCRIPTION

One embodiment is a method, comprising: reacting an alkanol, oxygen,carbon monoxide, and a catalyst to form a mixture comprising a dialkylcarbonate, an alkyl chloroformate, hydrochloric acid, water, carbondioxide, and carbon monoxide; and removing alkyl chloroformate from saidmixture.

There is no particular limitation on the alkanol used in the method.Suitable alkanols include primary, secondary, and tertiary C₁-C₁₂alkanols, with primary C₁-C₆ alkanols being preferred. Highly preferredalkanols include methanol.

Oxygen may be provided in any form, with gaseous forms being preferred.Suitable oxygen sources include, for example, air, and oxygen-containinggases having at least about 95 weight percent molecular oxygen,preferably at least about 99 weight percent molecular oxygen. Suitableoxygen-containing gases are commercially available from, for example,Air Products.

Carbon monoxide is preferably supplied as a gas having at least about 90weight percent, preferably at least about 95 weight percent, morepreferably at least about 99 weight percent, carbon monoxide. Suitablecarbon monoxide-containing gases are commercially available from, forexample, Air Products.

Suitable catalyst include those comprising iron, copper, nickel, cobalt,zinc, ruthenium, rhodium, palladium, silver, cadmium, rhenium, osmium,iridium, platinum, gold, mercury, and the like, and combinationscomprising at least one of the foregoing metals. Preferred catalysts maycomprise copper. A highly preferred catalyst comprises copper andchloride ion in a molar ratio of about 0.5 to about 1.5. Within thisrange, a molar ratio of at least about 0.8 may be preferred. Also withinthis range, a molar ratio of up to about 1.2 may be preferred. Highlypreferred catalysts include cuprous chloride (CuCl) and cupric chloride(CuCl₂), with cuprous chloride being more highly preferred. Duringoperation of the process, a suitable chloride ion concentration may bemaintained by the addition of hydrochloric acid (HCl).

FIG. 1 illustrates a dialkyl carbonate plant 10 having linked reactionsection 20, separation section 30, and purification section 40. Withreference to FIG. 1, the catalyzed reaction of alkanol, oxygen, andcarbon monoxide may be performed in a single reactor 50, or in two ormore reactors 50. The conditions for performing this step should beselected to maximize the yield of dialkyl carbonate while minimizing thedegradation of dialkyl carbonate. Preferably, the reaction is performedin a single reactor 50, at a temperature of about 50° C. to about 250°C. Within this range, the temperature may preferably be at least about100° C. Also within this range, the temperature may preferably be up toabout 150° C. The reactor 50 is preferably kept at a pressure of about15 to about 35 bar gauge (barg). Within this range, a pressure of atleast about 20 barg may be preferred. Also within this range, a pressureup to about 28 barg may be preferred. In the case of dual reactorsystems, the catalyst may be recycled between tanks. The catalystconcentration should be sufficiently high to produce an acceptableyield, but should be kept below a concentration that would cause solidsetting of the catalyst in the reactor 50 or clogging of the equipment.The reactants alkanol, oxygen, and carbon monoxide are preferably addedin a molar ratio of (about 0.5 to about 0.7):(about 0.04 to about 0.06):(about 0.8 to about 1.2), respectively. A highly preferred molar ratioof alkanol:oxygen:carbon monoxide is (about 0.6):(about 0.05):(about 1).

The amount of catalyst used relative to the reactants will depend on theidentity of the catalyst. For example, when the catalyst comprises CuCl,a highly preferred catalyst concentration is about 140 to about 180grams per liter of reaction mixture. During operation, the catalyst mayinitially be added from a catalyst tank 60. Sufficient HCl is preferablyadded to reactor 50 from a hydrochloric acid tank 70 during the courseof the reaction to maintain a molar ratio of Cu:Cl close to 1.0. Theconcentration of HCl is preferably continuously determined andcontrolled by the addition of HCl. A typical mass ratio for HCl feed tototal liquid feed is about 6×10⁻⁴ to about 8×10⁻⁴.

The reaction produces a mixture comprising a dialkyl carbonate, an alkylchloroformate, hydrochloric acid, water, carbon dioxide, and carbonmonoxide. The mixture may further comprise residual methanol and oxygen,as well as side-products such as alkyl chlorides and dialkyl ethers. Themixture is typically withdrawn from the reactor 50 in a gas/vapor form.The term “vapor” is meant to refer to gaseous organic components of themixture such as, for example, evaporated dialkyl carbonates, alcohols,alkyl chloroformates, etc., and to water vapor. That is, the term“vapor” refers to fluids having a boiling point of at least −50° C. atone atmosphere. In contrast, the term “gas” is meant to refer to thegaseous oxygen, carbon dioxide, carbon monoxide, and optional nitrogen.That is, the term “gas” refers to fluids having a boiling point lessthan −50° C. at one atmosphere. The vapor may be at least partiallycondensed in condenser 80, and fed to a first gas-liquid separator 90.The apparatus may optionally employ a single gas-liquid separator, or aplurality of (i.e., at least 2; preferably up to about 5) gas-liquidseparators. The first gas-liquid separator 90 may be kept at a pressurewithin about 10%, more preferably within about 1%, of the pressure ofthe reactor 50. The gas effluent from the first gas-liquid separator 90may be recycled, for example to reuse excess carbon monoxide. Themixture may be sent to a second gas-liquid separator 100, whichpreferably has a pressure less than about 20% of the pressure of thereactor 50 (e.g., preferably less than 3 bar gauge, more preferablyabout 0.2 bar gauge) to preferably achieve separation of at least about90%, more preferably at least 95%, by weight of the remaining gas in themixture. In a highly preferred embodiment, substantially all of the gasis removed from the mixture. The gas effluent removed from the secondgas-liquid separator 100 can also be recycled. It is preferred that thevapor in the mixture be in a partially condensed form (i.e., at leastabout 10% condensed), more preferably a fully condensed form (i.e., atleast about 90% condensed), before entering the first gas-liquidseparator 90, and between the first gas-liquid separator 90 and thesecond gas-liquid separator 100.

In the embodiment shown in FIG. 1, the mixture exiting the secondgas-liquid separator 100 may be in a single liquid phase. After thesecond gas-liquid separator 100, the mixture may proceed through a fluidpassageway 110 that removes alkyl chloroformate from the mixture. Itwill be understood that the terms “remove” and “removal” in reference toa particular chemical species encompass any chemical or physical processthat reduces the concentration of the species in the mixture. The alkylchloroformate may be removed from the condensate by any method. Somepreferred methods include heating, increasing pressure, increasingresidence time, adding a polar solvent, adsorbing, separating with amembrane (including gas and liquid membrane separation), pervaporating,passing through an ion exchange resin, exposing to a stoichiometricreagent, exposing to a catalytic reagent, and the like, and combinationscomprising at least one of the foregoing techniques. In a preferredembodiment, the alkyl chloroformate is removed from mixture by reactionwith water (see Scheme IV) or alkanol (see Scheme V).

It may also be preferred to remove the alkyl chloroformate withoutpassing the mixture through an ion exchange resin, because such resinsare expensive to install and operate. It may be preferred to remove atleast about 50 percent, more preferably at least about 90 percent, yetmore preferably at least about 95 percent, even more preferably at leastabout 99 percent, of the alkyl chloroformate from the mixture. In oneembodiment, it may be preferred to reduce the alkyl chloroformateconcentration in the mixture to less than about 500 ppm, more preferablyless than about 100 ppm, yet more preferably less than about 30 ppm. Inany of these embodiments, it may be preferred to remove less than about10%, more preferably less than about 5%, yet more preferably less thanabout 1%, of the dialkyl carbonate. Although the method may be describedas “removing less than about 10% of said dialkyl carbonate”, it will beunderstood that the concentration of dialkyl carbonate need not bereduced and may even increase. For example, the concentration of dialkylcarbonate may increase if the Scheme V reaction of alkyl chloroformatewith methanol forms dialkyl carbonate faster than dialkyl carbonatedecomposes due to other reactions.

Through extensive kinetic studies of the dimethyl carbonate processutilizing variations in factors including temperature, residence time,water concentration, methanol concentration, and hydrochloric acidconcentration, the present inventors have found that the rate of methylchloroformate decomposition may be given by the equation (1)

−r _(MCF)=(k _(i)[H₂O]+k ₂[MeOH])[MCF]  (I)

where r_(MCF) is the rate of change of the moles of methyl chloroformate(MCF) per unit volume, [H₂O], [MeOH], and [MCF] are the instantaneousconcentrations of water, methanol, and methyl chloroformate,respectively, in moles per unit volume, and k₁ and k₂ are rate constantsthat vary with temperature according to equations (2) and (3),respectively

k ₁ =k ₁ ⁰ e ^(−6381/T)  (2)

k ₂ =k ₂ ⁰ e ^(−7673/T)  (3)

where k₁ ⁰=2.09×10⁹ mL/mol-min, k₂ ⁰=4.14×10¹⁰ mL/mol-min, and T is thetemperature in degrees kelvin.

In many cases, it is valid to assume that the concentrations of waterand methanol, and the density of the solution are essentially constant.Within these general kinetic constraints, different kinetic expressionsmay be used for different process and apparatus types. With knowledge ofthe relevant chemical reactions and rate constants provided in thisapplication, these expressions may be readily derived by those ofordinary skill in the art. For example, in a batch process, the rate ofmethyl chloroformate decomposition may be expressed as a function ofresidence time, as shown in equation (4):

−d[MCF]/dt=(k ₁[H₂O])[MCF]  (4)

where t is residence time in minutes. The residence time t may bedefined as the total time spent by an average molecule in the fluidpassageway 110. In a batch process, at least about 50% of the methylchloroformate may be removed by maintaining the mixture under conditionscomprising a water concentration ([H₂O]), a methanol concentration([MeOH]), a temperature (T), and a residence time (t), such that aparameter X according to the equation (5)

X=exp{−[(2.09×10⁹)e ^(−6381/T)[H₂O]+(4.14×10¹⁰)e^((−7673/T))[MeOH]]t}  (5)

has a value less than about 0.9, wherein the water concentration and themethanol concentration are expressed in moles per milliliter, thetemperature is expressed in degrees Kelvin, and the residence time isexpressed in minutes. The value of X may preferably be less than about0.5, more preferably less than about 0.2, yet more preferably be lessthan about 0.1, even more preferably less than about 0.05, still morepreferably less than about 0.01. The water concentration may be about0.1 to about 50 moles per liter (mol/L). Within this range, the waterconcentration may preferably be at least about 0.5 mol/L, morepreferably at least about 1 mol/L. Also within this range, the waterconcentration may preferably be up to about 30 mol/L, more preferably upto about 20 mol/L, yet more preferably up to about 10 mol/L, even morepreferably up to about 5 mol/L. The methanol concentration may be about1 to about 25 mol/L. Within this range, the methanol concentration maypreferably be at least about 5 mol/L, more preferably at least about 10mol/L. Also within this range, the methanol concentration may preferablybe up to about 20 mol/L, more preferably up to about 18 mol/L. Theresidence time may be about 0.5 hour to about 10 hours. Within thisrange, the residence time may preferably be at least about 1 hours, morepreferably at least about 2 hours. Also within this range, the residencetime may preferably be up to about 8 hours, more preferably up to about6 6 hours. The temperature may be about 30 to about 130° C. Within thisrange, the temperature may preferably be at least about 40° C., morepreferably at least about 50° C., yet more preferably at least about 60°C. Also within this range, the temperature may preferably be up to about110° C., more preferably up to about 100° C., yet more preferably up toabout 90° C.

In the limit of an ideal steady state plug flow reactor, and assumingconstant density of the mixture, the rate of methyl chloroformatedecomposition may be expressed according to equation (3), with trepresenting residence time in minutes.

For an ideal steady state continuous stirred tank reactor (CSTR), theconcentration of methyl chloroformate is given by equation (6)

[MCF]=[MCF]_(t=0)(1/(1+kt))  (6)

where [MCF]_(t=0) is the initial concentration of methyl chloroformatein moles per milliliter, t is the residence time in minutes, and k isgiven by equation (7)

 k=k ₁[H₂O]+k ₂[MeOH]  (7)

where k₁, k₂, [H₂O], and [MeOH] are as defined above.

In another embodiment that relates to a batch reactor, removing alkylchloroformate from the mixture comprises maintaining the mixture underconditions comprising an initial concentration of methyl chloroformate([MCF]_(t=0)), a water concentration ([H₂O]), a methanol concentration([MeOH]), a temperature (T), and a residence time (t), such that aparameter Z calculated according to the equation (8)

Z=[MCF]_(t=0)exp{−[(2.09×10⁹)e ^((−6381/T))[H₂O]+(4.14×10¹⁰)e^((−7673/T))[MeOH]]t}  (8)

has a value less than about 5×10⁻⁶, preferably less than about 1×10⁻⁶,more preferably less than about 5×10⁻⁷, even more preferably less thanabout 5×10⁻⁸, wherein the initial concentration of methyl chloroformate,the water concentration, and the methanol concentration are expressed inmoles per milliliter, the temperature is expressed in degrees Kelvin,and the residence time is expressed in minutes. The temperature,residence time, methanol concentration, and water concentration in thisexpression are as described above. The initial concentration of methylchloroformate will depend on the reactor conditions, but it is typicallyabout 5×10⁻³ moles per liter to about 5×10⁻¹ moles per liter. Withinthis range, the initial concentration of methyl chloroformate may be atleast about 1×10⁻² moles per liter. Also within this range, the initialconcentration of methyl chloroformate may be up to about 1×10⁻¹ molesper liter.

In a preferred embodiment that relates to a batch reactor, removingalkyl chloroformate comprises subjecting the mixture to conditionscomprising an initial dimethyl carbonate concentration ([DMC]_(t=0)), aninitial water concentration ([H₂O]_(t=0)), an initial methanolconcentration ([MeOH]_(t=0)), an initial hydrochloric acid concentration([HCl]_(t=0)), a temperature (T), and a residence time (t), such that aparameter X calculated according to the equation (9)

X=exp{−[(2.09×10⁹)e ^((−6381/T))[H₂O]_(t=0)+(4.14×10¹⁰)e^((−7673/T))[MeOH]_(t=0) ]t}  (9)

has a value less than about 0.9, and a parameter Y calculated accordingto the equation (10) $\begin{matrix}{Y = \frac{\left( {1 - \frac{\left( \left\lbrack {H_{2}O} \right\rbrack \right)_{t = 0}}{\left( \lbrack{DMC}\rbrack \right)_{t = 0}}} \right)}{\left( {1 - {\left( \frac{\left( \left\lbrack {H_{2}O} \right\rbrack \right)_{t = 0}}{\left( \lbrack{DMC}\rbrack \right)_{t = 0}} \right)\left( {\exp \left( {\left( {6.6 \times 10^{10}} \right){{\left( {\exp \left( {{- 6636}/T} \right)} \right)\lbrack{HCl}\rbrack}_{t = 0}\lbrack{DMC}\rbrack}_{t = 0}\quad \left( {\frac{\left( \left\lbrack {H_{2}O} \right\rbrack \right)_{t = 0}}{\left( \lbrack{DMC}\rbrack \right)_{t = 0}}\quad - 1} \right)t} \right)} \right)}} \right)}} & (10)\end{matrix}$

has a value of at least about 0.9, wherein the initial dimethylcarbonate concentration, the initial water concentration, the initialmethanol concentration, and the initial hydrochloric acid concentrationare expressed in moles per milliliter, the temperature is expressed indegrees Kelvin, and the residence time is expressed in minutes. Thevalue of Y may preferably be at least about 0.95, more preferably atleast about 0.99. Suitable analytical techniques to determine initialconcentrations of water, methanol, hydrochloric acid, and dimethylcarbonate in reaction mixtures are well known in the art. The term“initial concentration” refers to the concentration of a species beforeintentional removal of alkyl chloroformate. The initial water andmethanol concentrations are the same as the water and methanolconcentrations described above (under typical reaction conditions, thewater and methanol concentrations are large are essentially constantduring alkyl chloroformate removal). The initial dimethyl carbonateconcentration may be about 0.5 to about 10 mol/L. Within this range, theinitial dimethyl carbonate concentration may preferably be at leastabout 1 mol/L, more preferably at least about 2 mol/L. Also within thisrange, the initial dimethyl carbonate concentration may preferably be upto about 8 mol/L, more preferably up to about 6 mol/L. The concentrationof HCl in the mixture will depend on the type and concentration ofcatalyst employed. The initial hydrochloric acid concentration willdepend on the type and amount of catalyst, but it is typically about1×10⁻³ to about 2×10⁻¹ moles per liter. Within this range, the initialhydrochloric acid concentration may preferably be at least about 5×10⁻³,more preferably at least about 1×10⁻² mol/L. Also within this range, theinitial hydrochloric acid concentration may preferably be up to about1×10⁻¹ more preferably up to about 7×10⁻² mol/L.

The method may be operated, for example, in a batch, semi-batch, orcontinuous manner.

In the particular embodiment shown in FIG. 1, the mixture passes througha first heat exchanger 140 to adjust the temperature of the mixtureabout 30° C. to about 130° C. Within this range, the temperature maypreferably be at least about 40° C., more preferably at least about 50°C. Also within this range, the temperature may preferably be up to about80° C., more preferably up to about 70° C. The term “heat exchanger”describes a well-known device for heating chemical reaction streams,typically by exchanging heat between a thermal energy source (e.g.,steam) and a cooler chemical reaction stream, but it is understood thatother types of equivalent heaters (e.g., electrical heaters) are alsoincluded. The condensate may proceed into a fluid passageway 110, whichserves to increase the residence time of the mixture under conditions tomaximize decomposition of alkyl chloroformate while minimizingdecomposition of dialkyl carbonate. The condensate may preferably remainfully condensed within the fluid passageway 110. It is desirable to keepthe condensate fully condensed because at least some alkylchloroformates (e.g., methyl chloroformate) are more stable in the vaporphase than the liquid phase under conditions used for this process.

The residence time and temperature in the fluid passageway 110 arepreferably sufficient to remove enough alkyl chloroformate to preventunacceptable downstream corrosion, but they should not be so excessiveas to cause unnecessary reductions in the productivity and yield of thedesired dialkyl carbonate product. FIG. 2 shows a simplified processdiagram representative of a comparison process. In this process, themixture flows directly from a first gas-liquid separator 90 to a firstheat exchanger 140, then to an acid removal column 160. Three specificembodiments of the fluid passageway 110 are shown in FIGS. 3, 4, and 5.In a preferred embodiment, at least about 50% of the alkyl chloroformateis removed, more preferably at least 80% is removed. In a highlypreferred embodiment, the alkyl chloroformate concentration is reducedto less than about 500 parts per million (ppm) by weight, morepreferably less than about 100 ppm by weight, yet more preferably lessthan about 30 ppm by weight, based on the total weight of the mixtureafter alkyl chloroformate removal. The fluid passageway 110 ispreferably selected such that the total residence time between thereactor 50 and the acid removal column 160 is about 0.5 hour to about 10hours. Within this range, the residence time may preferably be at leastabout 1 hour, more preferably at least about 2 hours. Also within thisrange, the residence time may preferably be up to about 8 hours, morepreferably up to about 7 hours.

In one embodiment, illustrated in FIG. 3, the fluid passageway 110comprises 2 holding vessels 120. These holding vessels 120 may, forexample, maintain the mixture at a temperature of about 55° C. for about2 hours. Each holding vessel 120 may preferably have a length to volumeratio (L/V) less than 5, preferably less than about 2. While two holdingvessels 120 are illustrated in this figure, there is no particularlimitation on the number of holding vessels 120 in the fluid passageway110. It may be preferred to use at least 2 holding vessels 120, andconfigurations comprising 3, 4, 5, 6, or more holding vessels 120 mayalso be preferred.

In another embodiment, illustrated in FIG. 4, the fluid passageway 110comprises 4 holding vessels 120. These holding vessels 120 may, forexample, maintain the mixture at a temperature of about 70° C. for about4 hours. Each holding vessel 120 may preferably have a length to volumeratio (L/V) less than 5, preferably less than about 2.

In yet another embodiment, illustrated in FIG. 5, the fluid passageway110 may comprise a section having L/V of at least 5, preferably at leastabout 10. For brevity, this section may be referred to as a tubularsection 130. Such a tubular section 130 having L/V>5 may promote plugflow of the mixture through the fluid passageway 110, therebyefficiently utilizing the residence time for removal of the alkylchloroformate. In this embodiment, it may be preferred that the mixturereside in one or more narrow sections having L/V>5 for at least about50% of the total residence time spent in the fluid passageway 110, morepreferably at least about 80% of the total residence time spent in thefluid passageway 110.

Referring again to FIG. 1, after exiting the fluid passageway 110, themixture may, optionally, pass through a second heat exchanger 150 to atleast partially vaporize the mixture. This second heat exchanger 150 mayhave a residence time of less than 10 minutes. This vaporization stepmay also be accomplished without a heat exchanger by lowering thepressure applied to the condensed mixture (e.g., by passing thecondensate into an acid removal column 160 that is kept at a relativelylower pressure). The vaporized mixture may then, optionally, be treatedto remove HCl, preferably by injecting it into an acid removal column160. The acid removal column 160 may also help remove any entrainedcatalyst (e.g., CuCl) that could otherwise contribute to downstreamcorrosion. In the acid removal column 160, the vaporized condensate maypreferably encounter a counter-flowing liquid supplied bycounter-flowing liquid line 170 to a higher point in the column (e.g.,the upper third). The counter-flowing liquid may trap the remaining HCland other reactants, which may be removed from the bottom of the acidremoval column 160 and recycled to the reactor 50. The dialkyl carbonatemixture may be removed from the top of the acid column 160 via exit line200, and, optionally, passed into an azeotrope column 180. As shown inFIG. 6, an optional ion exchange resin bed 190 may be included after theacid removal column 160, or at any other position downstream withrespect to the acid removal column 160. It may be advantageous toinclude an optional ion exchange resin bed 190 after water is removedfrom the product dialkyl carbonate stream in the purification section40. In a preferred embodiment, the apparatus does not include an ionexchange resin bed 190.

In a preferred embodiment, the method comprises reducing theconcentration of hydrochloric acid in the mixture to less than about1×10⁻³ mol/L, more preferably less than about 5×10⁻⁴ mol/L, even morepreferably less than about 1×10⁻⁴ mol/L, based on the total compositionafter removing hydrochloric acid.

In a preferred embodiment, the portions of the separation section 30downstream from the azeotrope column 180, and the purificationsubsection 40 are not required to be corrosion-resistant. Equipmentupstream of the azeotrope column 180 is preferably corrosion-resistant;for example, it may be glass lined. The term “corrosion-resistant” ismeant to describe a material capable of withstanding an HCl content of500 ppm at a temperature of about 50° C. to about 135° C. in thereaction mixture without substantial corrosion in a relatively brieftime period (e.g., six months). Glass lined vessels, precious metal(e.g., tantalum) lined vessels and special steels such as HASTELLOY® andCHROMALLOY® would be considered corrosion-resistant materials, whileordinary stainless steels not modified to enhance corrosion resistancewould not be considered corrosion-resistant. The azeotrope column 180can be made at least in part from corrosion-resistant metals. In apreferred embodiment, the bottom of the azeotrope column 180 may be madefrom a corrosion-resistant steel, whereas the top of the column can beordinary stainless steel.

In one embodiment of the apparatus, illustrated in FIGS. 1 and 3-6,alkyl chloroformate is removed in a fluid passageway 110.

In another embodiment of the apparatus, illustrated in FIG. 7, themixture is present in the gas-liquid separation vessels 90 and 100 forsufficient residence time and at sufficient temperature to remove alkylchloroformate. In other words, the fluid passageway 110 comprises thegas-liquid separation vessels 90 and 100. For example, the mixture mayremain in the condense phase in the gas-liquid separation vessels to besubstantially decomposed by reactions with water and methanol. In thisembodiment, the first heat exchanger 140 and the holding vessels 120 maybe unnecessary.

In another embodiment of the apparatus, illustrated in FIG. 8, the alkylchloroformate may be removed in a fluid passageway 110 that precedes thegas-liquid separation vessels 90 and 100. In this embodiment, one of theabove-mentioned techniques for removing alkyl chloroformate may beemployed upstream of the gas-liquid separation vessels 90 and 100.

In another embodiment of the apparatus, illustrated in FIG. 9, thehydrochloric acid may be removed from the mixture before removing thealkyl chloroformate. In this embodiment, the alkyl chloroformate may beremoved in the vapor, rather than the liquid phase. For example,referring to FIG. 9, the fluid passageway 110 may follow the azeotropecolumn 180; for example, it may be inserted into the azeotrope columnvapor exit line 210. In this embodiment, the first heat exchanger 140and the holding vessels 120 illustrated in FIG. 3 may be omitted. Inthis embodiment, the fluid passageway 110 may preferably comprise anapparatus suitable for removing alkyl chloroformate from the vapor phase(e.g., ion exchange resins, absorption beds, vapor phase membranes,etc.), and the alkyl chloroformate need not be condensed.

A preferred embodiment is a method of preparing a dialkyl carbonate,comprising: reacting an alkanol, oxygen, carbon monoxide, and a catalystto form a mixture comprising a dialkyl carbonate, an alkylchloroformate, hydrochloric acid, water, carbon dioxide, and carbonmonoxide; and passing said mixture through a fluid passageway 110 at atemperature of about 50° C. to about 80° C. and for a residence time ofabout 1 hour to about 10 hours.

Another preferred embodiment is an apparatus for preparing a dialkylcarbonate, comprising: means for reacting an alkanol, oxygen, carbonmonoxide, and a catalyst to form a mixture comprising a dialkylcarbonate, an alkyl chloroformate, hydrochloric acid, water, carbondioxide, and carbon monoxide; and means for removing alkyl chloroformatefrom said mixture.

Another preferred embodiment is an apparatus for preparing a dialkylcarbonate, comprising: a reactor for reacting an alkanol, oxygen, carbonmonoxide, and a catalyst to a produce a mixture comprising a dialkylcarbonate, an alkyl chloroformate, hydrochloric acid, water, and carbondioxide; and a fluid passageway 110 for removing alkyl chloroformate.

Dialkyl carbonates prepared according to the method are useful for thepreparation of diaryl carbonates. For example, diaryl carbonates may begenerated by the reaction of a dialkyl carbonate with an aryl hydroxide(see Scheme I, above). The diaryl carbonate may in turn be reacted witha dihydric phenol to form a polycarbonate (see Scheme II, above). Forexample, dimethyl carbonate prepared according to the method may bereacted with phenoxide to form diphenyl carbonate, which in turn may bereacted with bisphenol A to form a polycarbonate.

The invention is further illustrated by the following non-limitingexamples.

EXAMPLE 1

A plant according to simplified FIG. 2 was built and operated to producedimethyl carbonate. Corrosion damage was observed in and downstream ofthe azeotropic column 180. After extensive experimentation, it wasdetermined that the corrosion damage was caused by methyl chloroformatepassing through the acid separation column. Specifically, methylchloroformate was found to be present in the azeotrope column 180 at aconcentration of 300 parts per million (ppm) by weight.

EXAMPLES 2-5

The decomposition kinetics of methyl chloroformate were studied underfour different conditions. A procedure for determining methylchloroformate in a sample was as follows. For Example 2, 32 milliliters(mL) of dimethyl carbonate, 10 mL of dimethyl carbonate containing 50 mgof a biphenyl internal standard 63 mL of methanol, and 5 ml of waterwere added to a 250 mL flask equipped with a thermometer, a condenser,and a port for sampling. (Toluene may be used instead of themethanol/water solution.) The resultant homogeneous solution was placedin an oil bath and the temperature of the solution was held constant at50° C. At time zero, 81.7 microliters of pure methyl chloroformate wereadded to the solution (1,000 ppm on a weight basis). Samples werewithdrawn at various time intervals and were quenched by reacting themethyl chloroformate in the sample with diisobutyl amine to convert themethyl chloroformate to N,N′-diisobutyl methyl carbamate. The amount ofN,N′-diisobutyl methyl carbamate was then analyzed via titration with astandard silver nitrate solution to quantify the amount of ionicchloride present. The amount of methyl chloroformate could then beinferred by analyzing the original sample for ionic chloride. Thedifference in chloride concentration is equal to the methylchloroformate concentration because each equivalent of methylchloroformate liberates one equivalent of ionic chloride uponderivatization. Alternatively, gas chromatography can be used for directanalysis of the N,N′-diisobutyl methyl carbamate using an internalstandard.

Table I below show the observed decomposition rate constants (k) at 50°C. for various conditions. Example 2 corresponds to the case describedabove. Example 3 has added hydrochloric acid that is generally presentin the authentic reaction mixture. In Example 4, the effect of a smallamount of sodium bicarbonate was tested. In Example 5, the ratio ofdimethyl carbonate to methanol was held constant, but the amount ofwater was increased from 5% to 10%. The results are summarized below inTable I. [t1]

TABLE I DMC MeOH (wt %) (wt %) H₂O (wt %) Temp (° C.) k (min¹) Ex. 2 4550 5 50 0.043 Ex. 3* 45 50 5 50 0.043 Ex. 4** 45 50 5 50 0.480 Ex. 5***43 47 10 50 0.055 *Identical to Ex. 2, except that it also had 1000 ppmof HCl, which is similar to the effluent from the reactor 50.**Identical to Ex. 2, except that 1.6 eq. of NaHCO₃ relative to the 1000ppm MCF were added. ***Identical to Ex. 2, except that the % water wasincreased by 10%, but the ratio of DMC/MeOH was not changed, justreduced overall.

Plots of the logarithm of methyl chloroformate concentration versus timewere linear, fitting a pseudo-first-order kinetic model. This behaviorwas observed even in the presence of hydrochloric acid, and thereforethis method can be used to determine the concentration of methylchloroformate in a particular sample. Comparison of Examples 2 and 5indicates that only minor variations in the rate coefficient, k, areobserved when analyzing samples having water contents varying by afactor of two. Comparison of Examples 2 and 3 shows, surprisingly, thatadded HCl did not affect the observed rate of methyl chloroformatedecomposition. Comparison of Examples 2 and 4 shows that even a smallamount of base increased the reaction rate by more than ten-fold. As apractical matter, however, it may be desirable to avoid strongly basicconditions because they also may increase the decomposition rate ofdimethyl carbonate.

EXAMPLE 6 Comparative Example 1

These experiments show that the fluid passageway 110 is effective toreduce the concentration of methyl chloroformate that can react to formHCl in downstream sections of the plant. With reference to FIG. 1, twosamples were obtained by sampling the process fluid at different pointsin a dimethyl carbonate plant having a configuration with a first heatexchanger 140 and two holding vessels 120 (i.e., a configurationcorresponding to FIG. 3). The first sample (Comparative Example 1) wastaken immediately before the first heat exchanger 140. The second sample(Example 6) was taken after the second holding vessel 120 (i.e., afterthe fluid passageway 110). Each same was taken to the lab, and itschloride content was determined as a function of time elapsed fromsampling. The results are presented in Table II. The Ex. 6, data showessentially constant levels of chloride ion, indicating that labile,chloride-generating species such as methyl chloroformate are not presentin the sample. In contrast, the data for Comp. Ex. 1 show an increasewith chloride level over time, consistent with presence of methylchloroformate in the initial sample and its decomposition over time toform additional chloride ion. Thus, the data collectively show that inthe absence of the fluid passageway 110, substantial chloride formationmay take place in downstream (post-acid removal column 160) section ofthe plant, causing corrosion, but the presence of the fluid passageway110 is effective to decompose alkyl chloroformate to chloride ion beforethe acid removal column 160, thereby preventing downstream corrosion.[t2]

TABLE II Chloride Concentration (ppm) Time (h) Ex. 6 Comp. Ex. 1 0 374189 2 408 312 4 374 339 8 372 368 10 372 357 25 381 368

EXAMPLE 7 Comparative Example 2

For Comparative Example 2, a dimethyl carbonate plant according tosimplified FIG. 2 was operated according to the conditions described inTable III, below. This plant was similar to that shown in more detail inFIG. 1, with the exception that the first heat exchanger 140 and thefluid passageway 110 were absent. Corrosion was observed in anddownstream of the azeotrope column 180. Next, this plant was modified toinclude the first heat exchanger 140 and two holding vessels 120 wereadded to increase residence time (i.e., FIG. 3 configuration). FIG. 10presents measurements of residual ionic chlorides found in samples takenfrom the bottom of the azeotrope column 180, comparing the FIG. 2 andFIG. 3 configurations, each over time. Residual chlorides weredetermined by titration using a silver nitrate solution, as describedabove. The data for the FIG. 2 configuration have an average of 671 ppmchloride with a standard deviation of 370 ppm chloride, whereas the datafor the FIG. 3 configuration have an average of 35 ppm chloride and astandard deviation of 25 ppm chloride. The data thus show a dramaticreduction in chloride levels for the FIG. 3 configuration vs. the FIG. 2configuration. It is predicted this reduction would be even greater forthe configurations according to FIGS. 4 and 6, in which four holdingvessels 120 are used to provide a residence time of four hours at 70° C.FIG. 11 presents measurements of methyl chloroformate concentrationentering and exiting the fluid passageway 110 of the FIG. 3concentration. In other words, the points signified by “+” and labeled“MCF feed to Phase 0” in FIG. 11 correspond to measurements on themixture as it was entering the fluid passageway 110; these points havean average value of 930 parts per million by weight (ppmw) and astandard deviation of 412 ppmw. And the points signified by “▪” andlabeled “MCF from Phase 0” correspond to measurements on the mixture asit exits the fluid passageway 110; these points have an average value of45 ppmw and a standard deviation of 77 ppmw. These data clearly showthat an apparatus according to FIG. 3 is effective to dramaticallyreduce the concentration of methyl chloroformate in the process stream.[t3]

TABLE III Ex. 7 (FIG. 2 Control Ex. 2 (FIG. 3 Conditions Configuration)Configuration) Mass Ratio MeOH/O₂/CO 0.7/0.06/1 0.7/0.06/1 CatalystContent Fixed Fixed Reaction Temperature (° C.) 133 133 ReactionPressure (barg) 23 23 Temp. of Pre-Residence Time 60 — Heater (° C.)Temp. of Acid Column Feed 90 90 Vaporizer (° C.) Residence Time betweenflash 2 0.03 vessel and acid column, excluding both (hours)

TABLE IV average chloride concentration ± standard Configurationdeviation (ppm) FIG. 3 (comparison) 671 ± 370 FIG. 2 (invention) 35 ± 25

While the invention has been described with reference to a preferredembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing fromessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

Where not specifically defined herein, technical terms in thisspecification may be interpreted according to Grant and Hach's ChemicalDictionary, 5^(th) ed., McGraw-Hill, Inc.

All cited patents and other references are incorporated herein byreference in their entirety.

What is claimed is:
 1. A method of preparing a dialkyl carbonate,comprising: reacting an alkanol, oxygen, carbon monoxide, and a catalystto form a mixture comprising a dialkyl carbonate, an alkylchloroformate, hydrochloric acid, water, carbon dioxide, and carbonmonoxide; and removing alkyl chloroformate from said mixture.
 2. Themethod of claim 1, wherein said alkanol comprises a C₁-C₁₂ alkanol. 3.The method of claim 1, wherein said alkanol comprises a C₁-C₆ primaryalkanol.
 4. The method of claim 1, wherein said alkanol comprisesmethanol.
 5. The method of claim 1, wherein said alkanol, said oxygen,and said carbon monoxide are reacted in a molar ratio of (about 0.5 toabout 0.7 alkanol):(about 0.04 to about 0.06 oxygen):(about 0.8 to about1.2 carbon monoxide).
 6. The method of claim 1, wherein said catalystcomprises a metal selected from the group consisting of iron, copper,nickel, cobalt, zinc, ruthenium, rhodium, palladium, silver, cadmium,rhenium, osmium, iridium, platinum, gold, mercury, and combinationscomprising at least one of the foregoing metals.
 7. The method of claim1, wherein said catalyst comprises copper.
 8. The method of claim 1,wherein said catalyst comprises chloride ion.
 9. The method of claim 1,wherein said catalyst comprises chloride ion and copper in a molar ratioof about 0.5 to about 1.5.
 10. The method of claim 1, wherein saidreacting is performed in a single reactor.
 11. The method of claim 1,wherein said reacting is performed in a corrosion-resistant reactor. 12.The method of claim 1, further comprising removing carbon dioxide andcarbon monoxide from said mixture.
 13. The method of claim 12, whereinat least about 90% of said carbon dioxide and at least about 90% of saidcarbon monoxide are removed from said mixture.
 14. The method of claim12, wherein said removing carbon dioxide and carbon monoxide comprisespassing said mixture through a plurality of gas-liquid separationvessels.
 15. The method of claim 14, wherein said reacting is conductedat a first pressure, and said plurality of gas-liquid separation vesselscomprises a first gas-liquid separation vessel having a pressure withinabout 10% of said first pressure, and a second gas-liquid separationvessel having a pressure less than about 20% of said first pressure. 16.The method of claim 1, wherein at least about 50 weight percent of saidalkyl chloroformate is removed from said mixture.
 17. The method ofclaim 1, wherein at least about 90 weight percent of said alkylchloroformate is removed from said mixture.
 18. The method of claim 1,wherein at least about 95 weight percent of said alkyl chloroformate isremoved from said mixture.
 19. The method of claim 1, wherein at leastabout 99% of said alkyl chloroformate is removed from said mixture. 20.The method of claim 1, wherein said removing alkyl chloroformatecomprises removing less than about 5 weight percent of said dialkylcarbonate.
 21. The method of claim 1, wherein said removing alkylchloroformate comprises removing less than about 1 weight percent ofsaid dialkyl carbonate.
 22. The method of claim 1, wherein said removingalkyl chloroformate comprises reducing the concentration of said alkylchloroformate to less than about 500 parts per million by weight, basedon the total weight of said mixture.
 23. The method of claim 1, whereinsaid removing alkyl chloroformate comprises reducing the concentrationof said alkyl chloroformate to less than about 100 parts per million byweight.
 24. The method of claim 1, wherein said removing alkylchloroformate comprises reducing the concentration of said alkylchloroformate to less than about 30 parts per million by weight, basedon the total weight of said mixture.
 25. The method of claim 1, whereinsaid removing alkyl chloroformate comprises utilizing at least onetechnique selected from the group consisting of heating, increasingpressure, adding a polar solvent, adsorbing, separating with a membrane,pervaporating, passing through an ion exchange resin, exposing to astoichiometric reagent, exposing to a catalytic reagent, andcombinations comprising at least one of the foregoing techniques. 26.The method of claim 1, further comprising removing hydrochloric acid.27. The method of claim 26, wherein said removing hydrochloric acidcomprises reducing the concentration of said hydrochloric acid to lessthan about 1×10⁻³ moles per liter.
 28. The method of claim 26, furthercomprising vaporizing said mixture before said removing hydrochloricacid.
 29. The method of claim 28, wherein said vaporizing comprisesheating said mixture, reducing the pressure applied to said mixture, orboth.
 30. The method of claim 26, wherein said removing hydrochloricacid comprises passing said mixture through an acid removal column. 31.The method of claim 26, wherein said removing hydrochloric acidcomprises passing said mixture through an acid removal column and an ionexchange resin.
 32. The method of claim 1, wherein said method isoperated continuously.
 33. A method of preparing a dialkyl carbonate,comprising: reacting an alkanol, oxygen, carbon monoxide, and a catalystto form a mixture comprising a dialkyl carbonate, an alkylchloroformate, hydrochloric acid, water, carbon dioxide, and carbonmonoxide; reducing the concentration of said alkyl chloroformate in saidmixture to less than about 500 parts per million by weight, based on thetotal weight of said mixture, while removing less than about 10 weight %of said dialkyl carbonate; and removing hydrochloric acid from saidmixture.
 34. A method of preparing a dialkyl carbonate, comprising:reacting an alkanol, oxygen, carbon monoxide, and a catalyst to form afirst mixture comprising a dialkyl carbonate, an alkyl chloroformate,hydrochloric acid, water, carbon dioxide, and carbon monoxide; removingalkyl chloroformate from said first mixture to form a second mixture;and removing hydrochloric acid from said second mixture.
 35. The methodof claim 34, wherein said first mixture comprises a vapor of dialkylcarbonate and a vapor of alkyl chloroformate.
 36. The method of claim35, further comprising condensing said vapor of dialkyl carbonate andsaid vapor of alkyl chloroformate.
 37. The method of claim 36, whereinsaid condensing said vapor of dialkyl carbonate and said vapor of alkylchloroformate produces a single liquid phase.
 38. The method of claim34, wherein said removing alkyl chloroformate comprises using at leastone gas/liquid separator.
 39. A method of preparing a dialkyl carbonate,comprising: reacting an alkanol, oxygen, carbon monoxide, and a catalystto form a first mixture comprising a dialkyl carbonate, an alkylchloroformate, hydrochloric acid, water, carbon dioxide, and carbonmonoxide; removing hydrochloric acid from said first mixture to form asecond mixture; and removing alkyl chloroformate from said secondmixture.
 40. A method of preparing dimethyl carbonate, comprising:combining methanol, oxygen, carbon monoxide, and a copper catalyst toform a first mixture comprising a vapor of dimethyl carbonate, a vaporof methyl chloroformate, hydrochloric acid, water, carbon dioxide, andcarbon monoxide; removing a portion of said carbon dioxide and a portionof said carbon monoxide from said first mixture to form a secondmixture; at least partially condensing said vapor of dimethyl carbonateand said vapor of methyl chloroformate to form a third mixture; removingat least about 90weights % of said methyl chloroformate and less thanabout 1 weight% of said dimethyl carbonate from said third mixture toform a fourth mixture; and removing hydrochloric acid from said fourthmixture.
 41. A method of preparing a diaryl carbonate, comprisingreacting a dialkyl carbonate with an aryl hydroxide, wherein the dialkylcarbonate is prepared according to the method of claim
 1. 42. A methodof preparing a polycarbonate, comprising reacting a diaryl carbonatewith a dihydric phenol, wherein the diary carbonate is preparedaccording to the method of claim
 41. 43. A method of perparing dialkylcarbonate comprising: reacting alkanol, oxygen, carbon monoxide and acatalyst to form a first mixture comprising a vapor of alkyl carbonate,a vapor of alkyl chloroformate, hydrochloric acid, water, carbon dioxideand carbon monoxide; condensing the vapor of alkyl carbonate and thevapor of alkyl chloroformate and removing a portion of the carbondioxide and a portion of the carbon monoxide from the first mixture toform a fully condensed second mixture; and removing alkyl chloroformatefrom the fully condensed second mixture.
 44. The method of claim 1,wherein said removing alkyl chloroformate comprises increasing residencetime.